1. Field of the Invention
Embodiments described herein relate generally to MRI (magnetic resonance imaging) which magnetically excites nuclear spin of an object with an RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation.
Embodiments described herein also relate generally to technology for generating an intended RF output waveform in a magnetic resonance imaging apparatus and a magnetic resonance imaging method.
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which magnetically excites nuclear spin of an object set in a static magnetic field with an RF signal having the Larmor frequency and reconstructs an image based on NMR signals generated due to the excitation.
In the field of magnetic resonance imaging, technology of controlling a waveform of an RF transmission pulse is known. For example, a VFA (variable flip angle) method is known as technology for making a refocusing angle of an FSE (fast spin echo) sequence variable (for example, refer to Japanese Publication of Patent Application No. 2005-21690 and Japanese Patent No. 3405813). According to the VFA method, not only contrast of an image can be improved by adjusting a refocusing angle and thereby inhibiting blur, but also SAR (specific absorption rate) can be decreased. Note that SAR is a reference value showing accumulated energy amount in an object caused by a magnetic field.
In addition, a VERSE (variable rate selective excitation) method is known as technology of transforming a waveform of an RF transmission pulse. Moreover, a tag pulse which controls a waveform of an RF transmission pulse is known.
However, an RF amplifier overshoots or undershoots failing in generating a predetermined waveform of an RF transmission pulse precisely in some cases, when imaging is performed under a pulse sequence including an RF transmission pulse with a drastic gradient (slope) such as a square-wave pulse like a pulse sequence using the VFA method, the VERSE method or the tag pulse. Therefore, technology of generating an intended waveform of an RF output pulse steadily is demanded, when imaging is performed under application of an RF transmission pulse whose intensity changes rapidly.